Radar, lidar, and sonar systems are in widespread use for military, commercial, and private purposes. Radar systems have well-known characteristics, in that long-range detection of small targets is known to require transmission of more power, higher-gain antennas, and/or more sensitive receivers than that or those required for short-range detection of large targets. Lidar and sonar systems have equally well known characteristics. Among the characteristics of radar systems used for detecting targets at long range are those relating to range ambiguity, which has to do with reception of signals returned from a target lying beyond the range defined by the pulse repetition interval, which may make the distant target appear to be near the radar system. Another such characteristic of radar is that of range eclipsing, which has to do with the inability of a radar receiver to receive return signals during the pulse transmission interval.
Radar, lidar and sonar systems are used, among other purposes, for volume surveillance and target tracking in both commercial and military contexts. Ideally, a radar (lidar, sonar) system would detect targets at any range within selected limits, and provide information allowing determination of the range, velocity, and azimuth and elevation position of the target. There are well-known limitations to which such systems are subject, such that the maximum range is limited by the available transmit power and the sensitivity of the receiver. Pulsed radar systems transmit their power in pulses of given duration, and “listen” for return or reflected signals during inter-pulse periods. For a given maximum transmit power, the breaking up of the radar transmissions into pulses necessarily results in transmission of less than the maximum possible total power, as transmissions cease during the inter-pulse listening periods defined by the Pulse Recurrence Frequency (PRF) and the pulse duration. The Pulse Recurrence Frequency (PRF) is the inverse of the Pulse Recurrence Interval (PRI). In order to obtain maximum range, it is desirable to transmit the maximum available total power in a continuous-wave (100% of the time) manner.
A conventional solution to range eclipsing is to vary the pulse repetition interval, so that the transmitted pulses are staggered over time, thereby allowing the receiver to periodically “see” returned signals at times which would otherwise be lost or eclipsed. The eclipsing still occurs for each individual pulse train, but the totality of the radar returns over time includes information which fills in the gaps attributable to the individual transmitted pulse trains. The tradeoff is that a longer time is required to produce all the information required for an uneclipsed view of the region. Another possible solution to range eclipsing is to reduce the duty cycle of the radar by reducing the transmitted pulse duration, to thereby reduce the duration of the eclipsing.
The reduction of the pulse duration, however, tends to reduce the transmitted energy, which reduces the range sensitivity, which again requires a longer period of integration in order to obtain the same effective range.
Another possible solution to range eclipsing is to reduce the duty cycle of the radar by increasing the pulse repetition interval, to thereby move the increased range interval to a distant range not of interest. The reduction of the duty cycle and increase in the pulse repetition interval, however, tends to consume additional radar resources resulting in a greater overall time required for completion of a surveillance scan.
Conventional range ambiguity resolution techniques require transmission of additional signals with additional dwells for resolving the range interval of the ambiguous target. The additional dwells or transmissions consume additional radar resources, resulting in a greater overall time required for completion of a surveillance scan. U.S. Pat. No. 6,639,546, issued Oct. 28, 2003 in the name of Ott et al. describes a radar system which provides unambiguous and uneclipsed range by virtue of pulse-to-pulse frequency diversity, in combination with alternating interpulse intervals and processing which fills in target information in ranges which would otherwise be eclipsed by transmitted pulses. The pulse-to-pulse frequency diversity provides a tag for each pulse that allows the individual pulses to be separately identified.
Continuous-wave linear-frequency-modulation (CW-LFM) radars use LFM waveforms with 100% duty cycle. FIG. 1A is a simplified amplitude-range/time diagram illustrating the unambiguous range associated with a single pulse 10 transmission, as from a ship 12, with a given PRF. In FIG. 1A, the transmitted pulse is illustrated as 10, and the unambiguous range at which a return is received from a first target 22 is designated R1. A return from a second target 24 at a range greater than R1 also lies within the unambiguous range at a range R2. Both targets 20 and 22 lie within the unambiguous range since there is no transmission of a second pulse, or alternatively because the pulse recurrence interval extends beyond the maximum illustrated range.
FIG. 1B is a simplified amplitude-range/time diagram illustrating transmission of multiple pulses 10a, 10b, 10c, 10d, separated by a given pulse recurrence interval (PRI) equal to 1/PRF. In FIG. 1B, the unambiguous range extends from time 0 to time 1/PRF, which is the time at which transmission of the second pulse 10b begins. The first ambiguous range interval extends from time 1/PRF to time 2/PRF, the second ambiguous range interval extends from time 2/PRF to time 3/PRF, and the third ambiguous range interval extends from time 3/PRF to 4/PRF. Targets occurring at ranges within the ambiguous range intervals will appear to be closer to the ship 12 than their actual range. Thus, the reflected or return signal 22a from target 22 occurs within the first range ambiguity, so its range may be interpreted as less than its actual range. The reflected signal from target 24 does not occur within the first range ambiguity interval. The maximum unambiguous range is cT/2, where c is the speed of light and T is the pulse recurrence interval. The maximum unambiguous Doppler is ±λ/4T, where λ is the free-space wavelength. Also in FIG. 1B, the reflected signals 24a and 24b from target 24 occur in the second and third range ambiguity intervals, respectively, so its range may be interpreted as less than its actual range. In FIG. 1B, increasing the PRF increases the maximum unambiguous Doppler but decreases the maximum unambiguous range, and decreasing PRF increases the maximum unambiguous range, but decreases the maximum unambiguous Doppler.
FIGS. 1C and 1D are simplified amplitude/Doppler plots illustrating the measured Doppler of the returned target signals due to the transmitted pulse(s) of FIG. 1A and FIG. 1B, respectively. In both FIGURES, it can be seen that the unambiguous Doppler region extends from 0 to PRF*λ/2. Any target Doppler velocities outside this region will fold over and be ambiguous. In FIG. 1C, no Doppler response is shown, because a single pulse cannot extract radial velocity information from the return signal. In FIG. 1D, the radial velocities of the targets 22 and 24 are shown by 22E and 24E, respectively. The radial velocity of target 24 is less than PRF*λ/2, and thus its Doppler, marked as 24F, is unambiguous. The radial velocity of target 22, however, exceeds this limit, and its Doppler, marked as 22F, is folded over and ambiguous, therefore it may be interpreted as less than its actual Doppler.
FIG. 2 illustrates a frequency-time plot of three sequential linear-FM pulses. In FIG. 2, the frequency range extends from frequency −B/2 to frequency B/2.
Various frequency-modulation schemes have been developed to allow continuous-wave transmission.